Particle accelerators are typically constructed of a plurality of connected tubes which must be precisely aligned as they thread through magnets and are connected to radio frequency (RF) cavities. Precise alignment is necessary in order to not be hit by the particle beam they contain. Many accelerators connect the tubes by using CF flanged joints. The tubes are occasionally disassembled and reassembled for cleaning, routine maintenance, and upgrading of components. As the tubes are typically rigid metallic structures, a flexible mechanism must be provided to align the tubes in the correct position during assembly. Often the tubes are connected and at the same time constrained by the beamline components (magnets and cavities etc.) they are associated with. Subsequently the components are aligned and the tube joints must flex in order to accommodate the motion. Particle accelerators commonly use bellows as the mechanism to provide the flexure. With reference to FIGS. 1 and 2, a bellows 10, used in a particle accelerator, typically includes deep convolutions 12 of a thin material that enable the bellows to expand or contract in length and also provide angular flexibility. The bellows 10, typically constructed of metal, include end flanges 14 and a plurality of convolutions 16 extending between the end flanges.
There are several disadvantages that must be taken into account when using a bellows to connect beamline components in a particle accelerator. Often, bellows must be internally shielded to lower the beam impedance. Beam impedance is caused as the deeply convoluted surface of the inside of a conventional bellows interacts with the strong wake fields of pulsed beams. The convolutions reflect portions of the wake field that then cause degradation of the beam properties. A bellows shield is typically made of a thin copper based, metallic tube with a series of fingers that make electrical contact to the inside of the flanges at either end of the bellow such that the interior appears to the beam pulses to be electrically smooth and not cause wake field reflections. Unfortunately, especially in an SRF based accelerator, the spring fingers rubbing on the inside of the flanges tend to generate particulates that can migrate within the vacuum tubes into the superconducting cavities and reduce their performance. Thus, for an SRF-based accelerator, the flexible element of the beam tubes should not generate particles.
Further considerations in using bellows are their high cost, the need to supply adjustable support to the flanges at both ends in order to maintain the beam tube in the defined position, the difficulties in maintaining their cleanliness and the delicacy of the thin material used to make bellows that is subject to puncture.